There is a conventionally known electric power steering apparatus, according to which an extent of a steering force needed for operating a steering wheel can be reduced by applying an assist force by a motor to a steering mechanism connected to a steering shaft. In this type of electric power steering apparatus, a steering wheel is rotated, within a limited rotational range, more than one turn (cf. one turn is 360 degrees) in a clockwise direction and more than one turn (cf. one turn is 360 degrees) in a counterclockwise direction. When a vehicle runs straight ahead, the steering wheel is defined to be positioned at a neutral steering position. A steering angle of the steering wheel is grasped by detecting, as an absolute steering position, a rotational position of the steering wheel on a basis of a degree of angle from this neutral steering position.
The aforementioned steering angle of the steering wheel can be detected, for example, by a rotation angle sensor, which is configured with a slit disc rotatable together with a steering shaft, and a photo interrupter. For example, by employing a structure such as “a steering angle sensor and a power steering device” disclosed in pages 2-8 and FIGS. 1-4 of Japanese Laid Open Patent Publication No. 2002-145095A, it is possible to precisely detect a neutral steering position of a steering wheel by use of a single rotation angle sensor.
In general, a steering wheel of a vehicle is not capable of controlling, only by its one turn (cf. one turn is 360 degrees) or less, an angle of a steered wheel within an entire steered angle. For example, if the steering wheel is rotated, from a neutral steering position, two turns (cf. two turns are 720 degrees) in a clockwise direction and two turns (cf. two turns are 720 degrees) in a counterclockwise direction, i.e., if the steering wheel is rotated plus or minus 720 degrees from a neutral steering position, an actual angle of a steered wheel can be freely controlled within a predetermined steered angle. For this reason, even where a mechanism such as “the steering angle sensor and the power steering device” disclosed in JP2002-145095A is employed, it is not possible to detect an absolute steering position of a steering wheel only by use of a single rotation angle sensor. In the light of the foregoing, it is necessary to detect an absolute steering position of a steering wheel in combination with plural rotation angle sensors. Meanwhile, a structure, in which the multiple number of rotation angle sensors are provided, however, directly results in an increase in a manufacturing cost, and a failure rate, of a power steering device, such that it appears to be a reality that this type of structure, which may cause an increase in the number of components, is not favorable to be employed.
Another electric power steering apparatus has also been known, wherein there are employed a resolver as means for detecting the rotational position of an electric motor which generates the assisting force and another resolver as means for detecting the steering torque generated by a steering wheel. However, it was found through researches of the present inventor that it may occur independent on some angular positions of the steering wheel that the latter known electric power steering apparatus cannot precisely distinguish some absolute rotational positions from others precisely or definitely.